1. Field of the Invention
This invention relates to a copper alloy having high strength, high electrical conductivity and a resistance to stress relaxation at elevated temperatures. More particularly, the resistance to stress relaxation is enhanced by the presence of magnesium in solution with the copper.
2. Description of Related Art
Elemental copper has a very high electrical conductivity and relatively low strength and poor resistance to stress relaxation. Stress relaxation is an important consideration when selecting a copper alloy for an application where the product will be subjected to external stresses, such as when used for a spring or an electrical connector component.
Stress relaxation is a phenomenon that occurs when an external stress is applied to a piece of metal. The metal reacts by developing an equal and opposite internal stress. If the metal is restrained in the strained position, the internal stress decreases as a function of time. The gradual decrease in internal stress is called stress relaxation and happens because of the transformation of elastic strain in the metal to plastic, or permanent, strain. The rate of decrease of internal stress with time is a function of alloy composition, alloy temper, orientation and exposure temperature. It is desirable to reduce the rate of decrease, i.e. to increase the resistance to stress relaxation, as much as possible for spring and connector applications.
In the manufacture of an electrical connector, a sheet of copper alloy may be deformed into a hollow, generally cylindrical shape for use as a socket. Metal adjacent to an open end of the cylinder is externally stressed, such as by bending, to develop an opposing internal stress effective to cause the ends of the copper strip to bias inward and tightly contact a mating plug. This tight contact insures that the electrical resistance across the connector components remains relatively constant and that, in extreme conditions, the plug resists separation from the socket.
Over time, and more rapidly at higher temperatures, stress relaxation weakens the contact force between the socket and the plug and may eventually lead to connector failure. It is a primary objective of electrical connector design to maximize the contact force between the socket and the plug to maintain good electrical conductivity through the connector.
One copper alloy used to manufacture electrical connector components is designated by the Copper Development Association (CDA, Greenwich, Conn.) as copper alloy C19700. Copper alloy C19700 has the nominal composition, by weight, of 0.3%-1.2% iron, 0.1%-0.4% phosphorous, 0.01%-0.2% magnesium and the balance copper and unavoidable impurities.
Copper alloy C19700 has a resistance to stress relaxation that is marginal for many applications at exposure temperatures of 105.degree. C. or higher, particularly in the transverse orientation and for stronger tempers. It has been determined that after 3000 hours at an exposure temperature of 105.degree. C., a copper alloy C19700 connector in the hard temper, typically has about 64% stress remaining in the longitudinal direction and 56% stress remaining in the transverse direction.
The resistance to stress relaxation can be improved by a relief anneal. After the copper alloy sheet is rolled to final gage, it may be relief annealed for a hard temper by bell annealing at a strip temperature of from 200.degree. C. to 400.degree. C. for from 30 seconds to 4 hours. Strip annealing at corresponding higher temperatures and shorter exposure times is also useful. A connector formed from copper alloy C19700 in the hard/relief anneal temper typically has a longitudinal value of 72% stress remaining and a transverse value of 65% stress remaining after the same exposure to 105.degree. C. for 3000 hours.
Directionality is defined with reference to FIG. 1. A sheet 10 of a desired copper alloy is reduced in thickness by passing through the rolls 12 of a rolling mill. The copper alloy sheet 10 then has a longitudinal axis 14 along the rolling direction that is perpendicular to an axis 16 about which the rolls 12 rotate. The transverse axis 18 of the copper alloy sheet 10 is perpendicular to the longitudinal axis 14. Spring contacts formed from the copper alloy sheet and oriented parallel to the rolling direction are referred to as having a longitudinal (or good-way) orientation while spring contacts having an orientation transverse to the rolling direction are referred to as having a transverse (or bad-way) orientation.
United States patents that disclose a copper alloy containing iron, phosphorous and magnesium include U.S. Pat. No. 4,305,762 to Caron et al. and U.S. Pat. No. 4,605,532 to Knorr et al. Both of which are incorporated by reference in their entireties herein.
The Caron et al. patent discloses a copper alloy containing 0.04%-0.20% of magnesium, phosphorous and iron. The Knorr et al. patent discloses a copper alloy containing 0.01%-0.20% magnesium, 0.1%-0.4% phosphorous, 0.3%-1.6% iron and the balance copper. Published Japanese patent application No. JP 58-199835 by Sumitomo Electric discloses a copper alloy that contains 0.03%-0.3% magnesium, 0.03%-0.3% iron, 0.1%-0.3% phosphorous and the balance copper.
While copper alloys containing magnesium, phosphorous and iron are known, there remains a need for a copper alloy with an improved combination of electrical conductivity, strength and resistance to stress relaxation.